The use of stored or trapped charges is the dominant mechanism for storing data in a non-volatile memory cell. The charges of the memory cell are trapped or stored at various internal structures, such as: (a) floating gates; (b) material interfaces between, for example, a nitride and an oxide (SONOS cells); and (c) regions within an insulator that contain islands of conductive materials such as silicon rich oxides.
During device operation, these trapped charges function to modify, for example, the silicon surface conductivity of a FET. A corresponding general device configuration is illustrated in FIG. 1. A FET 100 includes a source 130 and a drain 120 separated by a control gate 110. This FET 100 can be of n-type or p-type. For simplicity we will describe an n-type FET. Thus, the drain 120 and the source 130 are both made of an n-doped semiconductor material while the substrate 140 is made of a p-doped semiconductor material. When a positive voltage is applied to the control gate 110, electrons within the substrate 140 are attracted toward the control gate 110 and form an inversion layer 150 in an area called the ‘channel’ that is within the substrate 140 and under the control gate 110. The channel then allows current conduction between the drain 120 and the source 130.
Notwithstanding the foregoing description, the presence of any trapped charges 160 within the oxide 170 and between the control gate 110 and the substrate 140 will modify the required value of the voltage applied to the control gate 110 to create the inversion layer 150. For a case of negative trapped charges 160, the higher the magnitude of trapped charge the higher the required voltage would have to be. The reason is that the negative charges trapped in the oxide will repel the electrons away from the channel, necessitating the application of a higher positive voltage on the control gate 110 to counteract this effect.
For each of the above devices, the magnitude of trapped charges can be altered through various mechanisms. Retrieving data can then be accomplished by sensing the voltage that is required to create the inversion layer.